Lithium ion cells are at present a large and growing market; they are very popular as electrochemical cells for portable electronic devices and lithium ion batteries are increasingly being applied to larger scale applications such as electric vehicles and stationary energy storage. They are a type of rechargeable cell in which lithium ions move from the negative electrode (anode) to the positive electrode (cathode) during discharge and back again when charging. During the charge or discharge, when the lithium ions are moving through the cell, a charge balancing current passes through an external circuit providing power.
However lithium is not a cheap metal to source and there is concern over its future availability and cost. Sodium-ion batteries are analogous in many ways to lithium ion batteries and research interest in sodium ion technology has increased over recent years. Although the sodium ions are larger and heavier than lithium ions, the same principle of the battery operation applies and the abundance and world wide access to sodium has led to predictions that sodium ion technology will provide a cheaper and more durable way to store energy in the future. This may be particularly applicable to large scale stationary energy applications.
Novel cathode and anode materials are being developed for sodium ion technology. The three main components of the cell are the cathode, anode and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is made from carbon. This is usually graphite for lithium ion cells; however for sodium ion technology the intercalation of the larger sodium ions into graphite is problematic, this is due to the size difference between the two ions, 95 pm for Na+ versus 68 pm for Li+. Typically a hard carbon (hard carbon is also known as amorphous carbon or disordered graphitic carbon and is distinguished from other carbons by having no long-range structure) is used instead, although other anode compositions are also under investigation. There are a wide range of possible cathode materials, and several examples in the prior art which describe the invention of novel layered oxide cathode materials for sodium ion batteries. For example, Komaba et al Adv. Funct. Mater. 2011, 21, 3859; Kim, Kang et al, Adv. Energy Mater. 2011, 1, 33-336, US2007/0218361, US20150243983A1.
During the first cycle of a rechargeable metal-ion cell different processes occur compared with subsequent cycles. On the anode side of the cell a layer known as the solid electrolyte interphase (SEI) is created. This SEI is formed at the negative electrode because some of the electrolyte components are not stable at the low voltages of this electrode during charging. The product of this decomposition forms a solid layer on the surface of the anode material. The chemical composition of this layer is very diverse and not thoroughly understood, and is dependent on the electrolyte used and surface properties of the anode. However, once this initial SEI layer has formed it can be impenetrable to the electrolyte molecules and electronically insulating and further significant build-up of the SEI is suppressed. Lithium ions however, can still pass through this layer to the active material. The formation of this SEI on the anode surface consumes some of the lithium which has originated from the cathode material. The lithium which forms the SEI layer is no longer available for shuttling between the cathode and anode and therefore the capacity of the cell is reduced on subsequent cycles compared with the first cycle. This is observed as a first cycle loss. The optimum procedure for the first cycle of the cell to establish a good quality SEI layer is dependent on the specific anode, cathode and electrolyte used and therefore is different for different cells.
U.S. Pat. No. 8,980,453 proposes overcharge during the formation charge of a cell but specifies that the cell is capacity limited by the negative electrode. This restricts the amount of sodium or lithium, and helps prevent plating of the metal onto the anode surface which can promote dendrite growth and lead to shorting of the cell. However, the requirement that the cell is capacity limited by the negative electrode means that the method of U.S. Pat. No. 8,980,453 is not widely applicable, and cannot be applied to many standard formats of cell as these have a cell capacity that is not limited by the cathode.
U.S. Pat. No. 8,168,330B2 proposes overcharge during the formation charge of a cell but is specific to lithium-ion cells. However, lithium-ion cells have different considerations to the sodium ion cells in an embodiment of the present invention and therefore the optimum procedures for sodium ion cells would be expected to be different.
US20150243983A1 describes an overcharge during the first charging cycle of a sodium ion battery. In this example the overcharging produces oxygen gas and the cell is degassed before continuing the cycling.